Hydrolytically Stable and Thermo-Mechanically Tunable Poly(Urethane) Thermoset Networks that Selectively Degrade and Generate Reusable Molecules

Cross-linked polymeric networks that possess tunable properties and degrade on-demand have broad applications in today’s society. Herein, we report on silyl-containing poly(urethane) (silyl-PU) thermoset networks, which are highly cross-linked stimuli-responsive materials with hydrolytic stability at 37.7 °C and 95% relative humidity, thermal stability of 280–311.2 °C, tensile properties of 0.38–51.7 MPa strength and 73.7–256.4% elongation, including storage modulus of 2268–3499 MPa (in the glassy state). However, unlike traditional (i.e., nondegradable) PU thermosets, these silyl-PUs selectively activate with fluoride ion under mild and static conditions to completely degrade, via cascading bond cleavages, and generate recoverable and reusable molecules. Silyl-PUs, as thin films, also demonstrated complete removal (within 30 min) from a strongly adhered epoxy thermoset network without altering the structure of the latter. Silyl-PU thermosets have potential applications in composite parts, vehicle and industrial coatings, and rigid plastics for personal devices, and may reduce environmental waste compared to nondegradable, single-use materials.

NMR spectra were performed on a Bruker 400 MHz NMR spectrometer and worked-up using Topspin. 13 C NMR spectra were recorded at 100 MHz. 19 F NMR spectra were recorded at 376 MHz. J coupling values are represented in Hz. NMR data is reported as follows: chemical shift (δ), multiplicity (bs = broad singlet, bt = broad triplet, singlet = singlet, d = doublet, t = triplet, q = quartet), coupling constant(s) in Hz, and integration. Highresolution mass spectroscopy (HRMS) was performed on a PerkinElmer AXION 2 time of flight (TOF) with direct sample analysis (DSA) source in positive mode.
Gel fraction analysis was performed on each sample by weighing, soaking in THF for 24 hours, drying in vacuo for 24 hours, and weighing again. Gel fraction was determined as the fraction of the final mass over the initial mass.
Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy was performed on a Nicolet iS50-FT-IR with iS50 ATR attachment equipped and a diamond ATR crystal from Thermo Scientific with 64 scans compiled for each spectrum. Spectra were recorded from 500-4000 cm -1 with a resolution of 2 cm -1 , and were analyzed using the Nicolet OMNIC software suite.
Thermal analysis was performed on TA Instruments Discovery Differential Scanning Calorimeter (DSC) to determine glass transition temperature (Tg). Two successive ramps were performed from -50 °C to 170 °C at a rate of 10 °C per minute, from which measurements were made on the second run. Samples were run in triplicate, or greater, and standard deviation was provided for each set. Thermogravimetric analysis (TGA) was performed on a TA Instruments Discovery TGA at a heating rate of 10 °C per minute under N2 from room temperature to 700 °C. Degradation onset temperature was assigned at the temperature at which 90% mass remained. TA Instruments Trios software was used to analyze DSC and TGA data.
Tensile testing of dog bone shaped samples (3.6 cm length x 1.4 cm width x about 1 mm thickness) was performed with a Texture Technologies TA.XT Plus Texture Analyzer at 21 °C to determine Young's modulus, ultimate tensile strength at break, and percent elongation at break. Samples were run at 10 mm per minute and had an area of 2.8 cm 2 . All samples were performed in at least triplicate to obtain standard deviation.
Dynamic mechanical analysis (DMA) of dog bond shaped samples was performed using a TA Instruments Q-800. The method equilibrated at -25 °C and held for 5 minutes, followed by a 5 °C ramp to 100 °C with a data acquisition of 1 Hz. Network Tg was calculated from the Tan Delta (Tan ) (E/E), which is the ratio of loss modulus (E) to the storage modulus (E).
Headspace sampling with gas chromatography and mass spectrometry (HS-GC-MS) analysis of partially degraded networks was performed with an Agilent 7696A Headspace Analyzer connected to an Agilent 7890B GC (with a Restek Rxi-5ms 15 m and 0.25 mm ID column) and Agilent 5977B MS. The headspace (HS) method used an oven temperature of 90 °C, a sample loop temperature of 90 °C, and a transfer line temperature of 110 °C. The vial of partially degraded network was equilibrated for 20 minutes before injection and the sample loop was filled for 0.5 minutes. The GC-MS method has an inlet temperature of 200 °C, initial oven temperature of 40 °C, which is held for 0.5 minutes, followed by a ramp of 20°C per minute to 250 °C, whereupon the temperature was held for 1 minute with a flow rate of 2 mL per minute. The mass spectrometer used electron impact ionization with a mass scan range of 30-500 m/z. After doing an initial scan the MS was changed to select ion monitoring (SIM) to provide a clearer chromatograph. The selected ions where 26 and 27 for ethylene, 44 for carbon dioxide (CO2), 77 and 162 for trifluorophenylsilane, and 101 for 3-methyl-2-oxazolidinone. For these experiments, 1.0 M TBAF in dimethylformamide (DMF) was used instead of THF to reduce solvent volatility and prevent obscuring molecules of interest.
Hygrothermal exposure of networks was performed using a Thermotron S4-8200 chamber at 37.7 °C and 95% relative humidity for 5 days, followed by drying of networks in vacuo at 60 °C for 24 hours.
Immersion experiments in stimuli solutions (e.g. fluoride salts) were conducted under static conditions at room temperature using pieces of networks with dimensions of approximately 9 mm length by approximately 6 mm width by approximately 1 mm thickness, unless otherwise noted.
Synthesis of extended chains silyl triols containing N-methylethanolamine was achieved using known methods to produce asymmetric carbamates through alkoxycarbonylation of an amine. 1

Phenyltrivinylsilane:
Phenyltrichlorosilane (8.6 mL, 53.0 mmol) was added to a 500 mL round bottom flask and cooled to 0 °C using an ice bath. A 1.0 M vinylmagnesium bromide solution in THF (185.5 mL, 185.5 mmol) was added to the flask while stirring. After 1 hour the cooling bath was removed and the mixture was stirred at room temperature for 16 hours. Saturated aqueous ammonium chloride (50 mL) was added, followed by dilution with deionized water (100 mL).
The resulting organic layer was isolated. The aqueous layer was extracted with dichloromethane (3 x 50 mL). The organic layers were combined and dried with MgSO4, filtered, and concentrated in vacuo to give an oil. Purification by column chromatography (hexanes) afforded the product as a colorless oil (7.9 g, 79.7% yield). 1  mmol) was then added to the flask with a stir bar and allowed to stir for 16 hours at room temperature. The reaction mixture was concentrated in vacuo to afford a yellow oil. The oil was suspended in deionized water (100 mL) and extracted using CHCl3 (3 x 50 mL). The organic layers were combined and concentrated in vacuo to afford a yellow oil. The oil was dissolved in dry acetonitrile (300 mL). Triethylamine (27.8 mL, 199.5 mmol) was added to the flask, followed by N-methylethanolamine (10.7 mL, 132.8 mmol). The reaction mixture was stirred at room temperature for 16 hours. The reaction mixture was concentrated in vacuo to afford a yellow oil. Purification by column chromotography (9:1 CH2Cl2:CH3OH) afforded T1 as a yellow oil (8.9 grams, 59.5% yield). 1

2,2′,2′′-(Methylsilanetriyl)tris(ethan-1-ol) (T2):
A solution of 0.5 M 9-borabicyclo[3.3.1]nonane in THF (491 mL, 245.5 mmol) was added to a 1 L three-neck round bottom flask and cooled to 0 °C using an ice bath. Methyltrivinylsilane (10.6 mL, 65.4 mmol) was added dropwise to the flask over a period of 15 minutes after which the ice bath was removed and the reaction mixture was stirred for 4 hours. After 4 hours, water (50 mL) was added to the flask, followed by a 3.0 M aqueous sodium hydroxide solution (50 mL). Subsequently, Aqueous hydrogen peroxide solution (30 Wt.%, 50 mL) was added dropwise over 1 hour while being kept at 0 °C.
The reaction mixture was heated to reflux and stirred for 3 hours. Upon cooling to room temperature, the reaction mixture was extracted with ethyl acetate (150 mL). The organic layer was dried using magnesium sulfate and concentrated in vacuo to afford a clear oil. Purification by column chromatography on silica gel (9:1 CH2Cl2:CH3OH) furnished T2 as a colorless oil (11.0 g, 94.4% yield). 1

2,2′,2′′-(Phenylsilanetriyl)tris(ethan-1-ol) (T3):
A solution of 0.5 M 9-borabicyclo[3.3.1]nonane in THF (300.6 mL, 150.3 mmol) was added to a 1 L three-neck round bottom flask and cooled to 0 °C using an ice bath. Phenyltrivinylsilane (7.5 g, 40.1 mmol) was added drop-wise to the flask over a period of 15 minutes after which the ice bath was removed and the reaction mixture was stirred for 4 hours. After 4 hours, water (50 mL) was added to the flask, followed by a 3.0 M aqueous sodium hydroxide solution (50 mL).
Subsequently, aqueous hydrogen peroxide solution (30 Wt.%, 50 mL) was added dropwise at 0 °C for over a 1 hour period. The reaction mixture was heated to reflux and stirred for 3 hours. Upon cooling to room temperature, the reaction mixture was extracted with ethyl acetate (150 mL). The organic layer was dried using magnesium sulfate and concentrated in vacuo to afford a clear oil. Purification by column chromatography on silica gel (9:1 CH2Cl2:CH3OH) furnished T3 as a colorless oil (7.6 g, 76.0% yield). 1         The 1 H and 13 C NMR spectra are shown in Figure S9 and S10.
Silyl-containing poly(urethane) N3 was synthesized by adding 2,2′,2′′-(phenylsilanetriyl)tris(ethan-1-ol) (2.00 grams, 0.0249 moles OH) (T3) to a 25 ml round bottom flask, followed by the addition of dry ethyl acetate (2.00 grams). Next, 1,6-hexamethylene diisocyanate (2.00 grams, 0.0238 moles NCO) was added, followed by stirring and heating at 60 °C for 1 hour until the mixture became clear. The solution was poured into a 2.5-inch aluminum weighing pan, the solvent was allowed to evaporate for 30-60 minutes, then the pan was placed in the oven at 60 °C for 36 hours to form the solid network. Network thickness was 1-2 mm.
Silyl-containing poly(urethane) N7 was synthesized by adding 2,2′,2′′-(phenylsilanetriyl)tris(ethan-1-ol) (2.00 grams, 0.0249 moles OH) (T3) to a 25 ml round bottom flask, followed by the addition of dry ethyl acetate (2.16 grams). Next, 1,6-hexamethylene diisocyanate (2.19 grams, 0.0261 moles NCO) was added, followed by stirring and heating at 60 °C for 1 hour until the mixture became clear. The solution was poured into a 2.5-inch aluminum weighing pan, the solvent was allowed to evaporate for 30-60 minutes, then the pan was placed in the oven at 60 °C for 36 hours to form the solid network. Network thickness was 1-2 mm.
Next, 1,6-hexamethylene diisocyanate (HDI) (12.8 grams, 0.152 moles NCO) was added, followed by 10 Wt.% solution of DBTDL in dry ethyl acetate (0.10 grams). The solution was then stirred by hand for 10-15 minutes until the solution became warm. The solution was poured into a 2.5-inch aluminum weighing pan and allowed to cure under ambient conditions for 24 hours. The thickness of the silyl ether poly(urethane) network was 2-3 mm ( Figure S26).

Synthesis of Epoxy-Amine Network
Epoxy-amine networks were made by adding liquid Bisphenol A / epichlorohydrin difunctional epoxy resin (Epon 828, Miller-Stevenson) (39.73 grams, 0.0421 grams per epoxy equivalence) to a plastic cup, followed by addition of xylenes (2.50 or 5.50 grams) and tris(2-aminoethyl)amine (10.27 grams, 0.0422 grams per NH equivalent). The mixtures were stirred with a magnetic stir bar for 20 minutes, followed by spin-coating the dilute solution on the gold slides and applying the concentrated solution on the chromic acid anodized aluminum alloy panels via a 3 mil (76.2 micron) film forming bar. The gold slides were placed in the oven at 60 °C for 24 hours, whereas the aluminum panels were allowed to cure under normal laboratory conditions for 24 hours. The film thickness of the cured epoxy-amine network on the gold slides was 10-15 microns, whereas the cured thickness on the pretreated aluminum panels was an average of 40 microns.

Synthesis of Poly(urethane) Networks (N1-N5) over Epoxy-Coated Gold Slides
Non-silyl and silyl-containing poly(urethane) networks N1-N5 over epoxy-coated gold slides were formed by remaking the network formulas according to the aforementioned procedures, but instead of decanting into an aluminum pan they were spin-coated on the 24-hour cured epoxy-coated gold slides. The samples were then placed in the oven at 60 °C for 24-hours. The resulting film thickness of the poly(urethane) networks was about 10-15 microns.

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